It is known that many microorganisms synthesize poly(3-hydroxybutanoic acid) (hereinafter sometimes referred to as "PHB") as an intracellular reserve substance which is accumulated in the form of granules in the cytoplasm (Anderson, A. J. And Dawes, E. A., Microbiol. Rev., 54: 450-472 (1990)). Recently, attention has been paid to PHB isolated from microbial cells as thermoplastic resin having biodegradability and biocompatibility (Doi, Y., In: Microbial polyesters. VCH. New York (1990)) and its application to formings such as films or fibers has been studied.
However, the conventional PHB homopolymer has not yet been put into practical use since it is physicochemically disadvantageous in its hardness and brittleness and is difficult to be used as formings by itself. On the other hand, it was found that the combination use of a 3-hydroxybutanoic acid with a copolymer component such as 3-hydroxypentanoic acid provides a flexible material having improved impact resistance and therefore, an attempt has been made to process copolymers that vary in their compositions into flexible films (see Holmes, P. A., Phys. Technol., 16, p32-36 (1985) and "Biodegradable Polymer Materials" Y. Doi ed., (Industrial Examination Committe, 1990) p. 27). However, methods of adding such a copolymer component may possibly be accompanied by the increased production cost, which would make economical performance worse. It was reported that an attempt was made to produce a film having high strength by subjecting PHB homopolymer to a orienting treatment, but not only it requires complicated conditions and a process having a number of steps but also it is not reproducible (Holmes, P. A., In: Developments in Crystalline Polymers-2, Bassett, D. C. (ed.), p1-65, Elsevier (1988)).
Accordingly, it has been desired to develop a method of simply and reproducibly producing a PHB film having improved physicochemical properties with retaining biodegradability.